doc: document best practices for main.c suite declaration
doc: document best practices for main.c suite declaration

With the introduction of --list-tests to sanitycheck, a few rules have
been added to how to declare test suites.

Signed-off-by: Inaky Perez-Gonzalez <96af2f95d738a33e3cb28686ea74cc090643af9b@intel.com>

ldts/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,finikorg/zephyr,finikorg/zephyr,punitvara/zephyr,galak/zephyr,mbolivar/zephyr,galak/zephyr,nashif/zephyr,explora26/zephyr,punitvara/zephyr,explora26/zephyr,Vudentz/zephyr,punitvara/zephyr,galak/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,mbolivar/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,zephyriot/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,Vudentz/zephyr,explora26/zephyr,zephyriot/zephyr,finikorg/zephyr,mbolivar/zephyr,mbolivar/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,zephyriot/zephyr,ldts/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,punitvara/zephyr,ldts/zephyr,mbolivar/zephyr,kraj/zephyr,nashif/zephyr,explora26/zephyr,finikorg/zephyr,kraj/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyriot/zephyr,kraj/zephyr,nashif/zephyr,zephyriot/zephyr,ldts/zephyr,Vudentz/zephyr